Can you give more information. What do you mean with "grub time" and what is your default os. In your grub menu what are the different lines?
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Julien ChauJun 8 '12 at 10:16

1

grub time means the countdown time when i have to select the os at the BIOS starting screen. I'm sorry but I don't know exactly what that os selection menu is called so I posted it as grub time.
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meteorsJun 8 '12 at 13:23

If you read the file that you are editing in the example above (/etc/default/grub), you will notice that the very first couple lines instruct you to run update-grub after making changes in order to update the actual file that grub reads to "get its instructions" (/boot/grub/grub.cfg). Note that you must actually run it with the sudocommand first as you need root privileges to actually run the command (which is why the poster above said to type sudo update-grub). This will cause the changes you made to be written to /boot/grub/grub.cfg. The very next couple lines tell you that you can read the full documentation of options in that file (again, /etc/default/grub) by typing info -f grub -n 'Simple configuration'.

That said, set GRUB_TIMEOUT to -1 if you want to set the "grub time" to be indefinite. In other words, it will never automatically boot. You will have to make a selection.

Finally, to answer your question, here are the descriptions of those "grub hidden lines" straight from the above-referenced documentation:

GRUB_HIDDEN_TIMEOUT
Wait this many seconds for a key to be pressed before displaying
the menu. If no key is pressed during that time, boot
immediately. Unset by default.
GRUB_HIDDEN_TIMEOUT_QUIET
In conjunction with `GRUB_HIDDEN_TIMEOUT', set this to `true' to
suppress the verbose countdown while waiting for a key to be
pressed before displaying the menu. Unset by default.